1. Technical Field
An embodiment of the invention pertains generally to the fabrication of surface elements on a substrate, and in particular pertains to the fabrication of surface grating structures using integrated circuit fabrication techniques.
2. Description of the Related Art
Optical gratings with a sub-micron pitch have multiple uses, among them as distributed Bragg reflectors (DBR) in edge-emitting lasers. In edge-emitting lasers, photons of light are generated in a layer of substrate material and emitted from an edge of the substrate. A grating pattern on a surface of the substrate is used to generate successive multiple in-phase reflections of the light that is generated within or guided by the substrate, permitting a particular wavelength of light to be reflected through constructive reinforcement. The grating pattern is implemented with parallel ridges (i.e., parallel lines of material along the surface that are raised above the material between ridges), with each ridge internally reflecting a portion of the light that strikes it. The grating pitch (the center-to-center spacing between adjacent ridges in the grating) determines the wavelength of the emitted laser light, and multiple grating sections with different pitches can be used to reflect multiple wavelengths of light at the same time, such as in the grating structures of a sampled grating, a superstructured grating, a binary grating, etc.
In order to optimize the performance of the optical fiber used in telecommunication, many edge-emitting lasers require a grating with a pitch of about 250 nanometers (nm). Conventional fabrication techniques to produce gratings with this pitch have at least one of these drawbacks: they are 1) too expensive (the cost per grating is too high), 2) too slow (the throughput is inadequate for volume production), 3) poor quality (the grating lines lack the necessary precision for satisfactory laser performance), and 4) cannot produce multi-pitch or circular gratings (needed for tunable semiconductor lasers and/or special-purpose applications).